Radio communication system and distance measuring method

ABSTRACT

A radio communication system, in which radio communication is carried out between an in-vehicle communication device, mounted in a vehicle, and a radio communication terminal to detect a relative distance therebetween, includes an output control unit that controls a radio wave intensity of a measuring signal, which is sent from one of the in-vehicle communication device and the radio communication terminal, at a pre-set predetermined value; and a measuring unit that performs distance measuring on a vehicle side based on the radio wave intensity of the measuring signal controlled at the predetermined value or on a signal to which a value of the radio wave intensity of the measuring signal is added as information.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radio communication system and adistance measuring method.

2. Description of the Related Art

As a system for carrying out radio communication between an in-vehiclecommunication device and a user-owned radio communication terminal, asystem is already known that can locks/unlocks the door of a vehiclewithout using a mechanical key. According to this system, when a usercarrying an electronic key approaches the vehicle, the in-vehiclecommunication device checks the ID code of the electronic key. When thevalidity of the electronic key is successfully authenticated as a resultof the checking and the electronic key enters a predetermined distancerange from the vehicle position, the vehicle can be unlocked withouthaving to insert the mechanical key into the cylinder lock of thevehicle. As an electronic key, though only a terminal conforming to thespecification of the in-vehicle communication device is used today,study has been conducted on the use of a user-owned general-purposeradio communication device.

Meanwhile, the relative distance not only between an in-vehiclecommunication device and a radio communication terminal such as anelectronic key but also between any two devices that carry out radiocommunication with each other is calculated in many cases based on thereception radio wave intensity of the radio signal. For example,Japanese Patent Application Publication No. 2005-328231 (JP 2005-328231A) discloses a system in which the relative distance is detected by aradio terminal device that wirelessly communicates with a fixed accesspoint. The radio terminal device receives a radio signal sent from theaccess point, detects its radio wave intensity and, based on thedetected reception radio wave intensity, estimates the distance to theaccess point. In addition, based on the distance estimated in thismanner, the radio terminal device determines the radio wave intensity ofradio communication via which it sends a signal and, at the same time,gradually increases the level of the determined sending radio waveintensity.

However, in a radio communication system composed of an in-vehiclecommunication device and a mobile radio communication terminal, thefollowing problem may arise. That is, when the in-vehicle communicationdevice measures the distance by communicating with the mobile radiocommunication terminal that has the function to make variable thesending radio wave intensity as with the device described in the abovedocument, the relative distance between them may be incorrectlydetected. In other words, it is difficult to measure the distance when ageneral-purpose radio communication terminal is used as a terminal thatwirelessly communicates with an in-vehicle communication device.

SUMMARY OF THE INVENTION

The present invention provides a radio communication system and adistance measuring method capable of measuring the distance to a radiocommunication terminal even when general-purpose radio communication isused.

A first aspect of the present invention relates to a radio communicationsystem in which radio communication is carried out between an in-vehiclecommunication device, mounted in a vehicle, and a radio communicationterminal to detect a relative distance therebetween. The radiocommunication system includes an output control unit that controls aradio wave intensity of a measuring signal at a pre-set predeterminedvalue, the measuring signal being sent from one of the in-vehiclecommunication device and the radio communication terminal; and ameasuring unit that performs distance measuring on a vehicle side basedon the radio wave intensity of the measuring signal controlled at thepredetermined value or on a signal to which a value of the radio waveintensity of the measuring signal is added as information.

A second aspect of the present invention relates to a distance measuringmethod for carrying out radio communication between an in-vehiclecommunication device, mounted in a vehicle, and a radio communicationterminal to detect a relative distance therebetween. The distancemeasuring method of the radio communication system includes controllinga radio wave intensity of a measuring signal at a pre-set predeterminedvalue, the measuring signal being sent from one of the in-vehiclecommunication device and the radio communication terminal; andperforming distance measuring on a vehicle side based on the radio waveintensity of the measuring signal controlled at the predetermined valueor on a signal to which a value of the radio wave intensity of themeasuring signal is added as information.

According to the aspects described above, the radio wave intensity ofthe measuring signal is controlled at a pre-set intensity and, then,distance measuring is performed based on the radio wave intensity of themeasuring signal or on a signal to which the radio wave intensity of themeasuring signal is added as information. This configuration thereforeallows distance measuring to be performed even for a radio communicationterminal that has the general-purpose communication function that makesvariable the sending radio wave intensity.

In the first aspect described above, the output control unit may send anoutput intensity maintaining request that requests a sending radio waveintensity of the radio communication terminal be maintained at apredetermined intensity and the measuring unit, provided in thein-vehicle communication device, may calculate a distance to the radiocommunication terminal based on a reception radio wave intensity of aresponse signal that is sent from the radio communication terminal withthe predetermined intensity.

In the second aspect described above, with the use of an output controlunit that requests that a sending radio wave intensity of the radiocommunication terminal be maintained at a predetermined intensity; and ameasuring unit that, provided in the in-vehicle communication device,performs distance measuring based on the radio wave intensity of themeasuring signal controlled at the predetermined value, the distancemeasuring method may include sending, by the output control unit, anoutput intensity maintaining request that requests that the sendingradio wave intensity of the radio communication terminal be maintainedat a predetermined intensity; sending, by the radio communicationterminal, a response signal with the sending radio wave intensitymaintained at a predetermined intensity; and calculating, by thedistance measuring unit, a distance to the radio communication terminalbased on a reception radio wave intensity of the response signal sentfrom the radio communication terminal.

According to the aspects described above, after the above-describedoutput control signal is received, the sending radio wave intensity ofthe radio communication terminal becomes constant. This configurationtherefore allows distance measuring to be performed even for a radiocommunication terminal that has the general-purpose communicationfunction that makes variable the sending radio wave intensity.

In the first aspect described above, the output control unit may sendthe output intensity maintaining request to an authenticated radiocommunication terminal. According to this aspect, the above-describedoutput control signal is sent only to an authenticated radiocommunication terminal. This configuration therefore preventsunnecessary output control from being performed for a radiocommunication terminal that is present around the vehicle but is notauthenticated.

In the first aspect described above, the output control unit, providedoutside the vehicle, may send the output intensity maintaining requestto all the radio communication terminals each of which hasidentification information indicating that the radio communicationterminal is capable of communicating with the in-vehicle communicationdevice.

According to this aspect, the above-described output control signal issent to all the radio communication terminals each of which has theabove-described identification information. This configuration thereforeprevents the request, which sets the sending radio wave intensity to apredetermined level, from being sent at least to a mobile radiocommunication terminal that has not the identification information.

In the first aspect described above, the output control unit, providedin the in-vehicle communication device, may send a request signal with apredetermined radio wave intensity, the request signal requesting for aresponse signal from the radio communication terminal and, when therequest signal is received from the in-vehicle communication device, theradio communication terminal may detect a reception radio wave intensityof the request signal and send the response signal as the measuringsignal, the response signal including the detected reception radio waveintensity as the information.

In the second aspect described above, with the use of an output controlunit that, provided in the in-vehicle communication device, sends arequest signal with a predetermined intensity, the request signalrequesting for a response signal from the radio communication terminal;and a distance measuring unit that, provided in the in-vehiclecommunication device, performs distance measuring based on a signal towhich the value of the radio wave intensity of the measuring signal isadded as the information, the distance measuring method may includesending, by the output control unit, the request signal with apredetermined intensity; detecting, by the radio communication terminal,a reception radio wave intensity of the request signal and then sendingthe response signal including the reception radio wave intensity; andcalculating, by the distance measuring unit, a distance to the radiocommunication terminal based on the reception radio wave intensityincluded in the response signal.

According to these aspects, the reception intensity of a radio wave sentfrom the in-vehicle communication device to the radio communicationterminal with a predetermined intensity is detected by the radiocommunication terminal and its intensity information is returned to thein-vehicle communication device. Therefore, distance measuring may beperformed on the vehicle side without having to control the sendingradio wave intensity of the radio communication terminal. Thisconfiguration therefore allows distance measuring to be performed evenfor a radio communication terminal that has the general-purposecommunication function that makes variable the sending radio waveintensity.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the invention will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a block diagram showing a general configuration of anin-vehicle radio communication system and a mobile radio communicationterminal in a first embodiment of a radio communication system and itsdistance measuring method of the present invention;

FIG. 2 is a schematic diagram showing a method of radio communicationbetween a radio communication system and a mobile radio communicationterminal in the embodiment;

FIG. 3 is a sequence chart showing a procedure of radio communicationperformed by an in-vehicle communication device and a mobile radiocommunication terminal in the embodiment;

FIG. 4 is a block diagram showing a general configuration of anin-vehicle radio communication system and a mobile radio communicationterminal in a second embodiment of a radio communication system and itsdistance measuring method of the present invention;

FIG. 5 is a schematic diagram showing a method of radio communicationbetween an in-vehicle communication device and a mobile radiocommunication terminal in the embodiment;

FIG. 6 is a sequence chart showing a procedure of radio communicationperformed by an in-vehicle communication device and a mobile radiocommunication terminal in the embodiment;

FIG. 7 is a block diagram showing a general configuration of anin-vehicle radio communication system and a mobile radio communicationterminal in a third embodiment of a radio communication system and itsdistance measuring method of the Present invention; and

FIG. 8 is a sequence chart showing a procedure of radio communicationperformed by an in-vehicle communication device and a mobile radiocommunication terminal in the embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

(First Embodiment)

A first embodiment is described below in which a radio communicationsystem for vehicles and the distance measuring method used in the radiocommunication system are embodied.

As shown in FIG. 1, a vehicle 100 in which the radio communicationsystem and the radio communication method, both specifically designedfor vehicles, are used is equipped with an electronic key system 110.This electronic key system 110 is used for radio communication with amobile radio communication terminal 11. The mobile radio communicationterminal 11 in this embodiment is a multi-function phone (smartphone)connectable to a general-purpose communication network with its functionvarying according to the machine type.

The electronic key system 110 includes an in-vehicle communicationdevice 10 that wirelessly communicates with the mobile radiocommunication terminal 11. The in-vehicle communication device 10includes an in-vehicle communication unit 12 that performs radiocommunication, an in-vehicle device 15 that acts as an output controlunit and a distance measuring unit, and an ID checking unit 16 thatauthenticates the mobile radio communication terminal 11. In thisembodiment, the in-vehicle communication unit 12, a communication unitfor radio communication according to a protocol based on the Bluetooth(registered trademark) standard, communicates with an external radiocommunication terminal conforming to the standard. The in-vehiclecommunication unit 12 includes an in-vehicle communication antenna 13and a reception intensity detection unit 14 that detects the receptionradio wave intensity of a signal sent from the mobile radiocommunication terminal 11. The in-vehicle communication unit 12 outputsa radio signal, the maximum communication distance of which is 10 m to100 m (for example, class 1 to class 2), to the in-vehicle communicationantenna 13.

The ID checking unit 16 checks the ID information, received from themobile radio communication terminal 11, with the registered IDinformation on the vehicle. The ID checking unit 16 outputs the checkingresult to the in-vehicle device 15. If it is determined as a result ofthe checking that the mobile radio communication terminal 11 isauthentic, the in-vehicle device 15 performs an operation, such as doorlocking/unlocking and engine start, in response to an operationperformed by the user.

As devices for locking and unlocking the door, the vehicle 100 includesan operation detection unit 17 that detects whether the user operatesthe door handle, a door lock control unit 18 that controls thelocking/unlocking of the door, and a door lock detection unit 19. Theoperation detection unit 17 detects that the user operates the doorhandle, provided outside the door of the vehicle 100, and Outputs thedetection information to the door lock control unit 18 and thein-vehicle device 15. The door lock detection unit 19 detects thelocking/unlocking of the door and outputs the detection information tothe door lock control unit 18 and the in-vehicle communication unit 12.

If the mobile radio communication terminal 11 is successfullyauthenticated, the in-vehicle device 15 sends an output intensitymaintaining request, which includes a command that maintains the outputto the antenna at a predetermined output level, to the authenticatedmobile radio communication terminal 11. This request causes theauthenticated mobile radio communication terminal 11 to control theoutput of the antenna at a predetermined output level, thus making itpossible to measure the distance based on the radio wave intensityreceived by the in-vehicle communication unit 12. That is, if the mobileradio communication terminal 11 is successfully authenticated and if thedistance from the vehicle 100 is the same, the reception radio waveintensity is the same regardless of the device type. Based on thereception radio wave intensity detected by the in-vehicle communicationunit 12, the in-vehicle device 15 references the table or the map, whichassociates reception radio wave intensity with a relative distance tothe communication terminal, to detect the relative distance to themobile radio communication terminal 11.

If the mobile radio communication terminal 11 is successfullyauthenticated and if the relative distance between the authenticatedmobile radio communication terminal 11 and the vehicle 100 is equal toor shorter than a predetermined distance, the in-vehicle device 15unlocks the door at the time the door handle is operated. Similarly, ifthe mobile radio communication terminal 11 is successfully authenticatedand if the relative distance between the authenticated mobile radiocommunication terminal 11 and the vehicle 100 is longer than apredetermined distance, the in-vehicle device 15 locks the door. Thedoor lock detection unit 19 detects the locking and unlocking of thedoor and sends the detection result to the mobile radio communicationterminal 11 via the in-vehicle communication unit 12.

In addition, the vehicle 100 includes an engine switch 20 that starts orstops the engine and an engine control unit 21 that controls the engine.If the mobile radio communication terminal 11 is successfullyauthenticated and if the relative distance between the authenticatedmobile radio communication terminal 11 and the vehicle 100 is equal toor shorter than a predetermined distance, the in-vehicle device 15outputs a start request to the engine control unit 21 at the time theengine switch 20 is turned on.

Next, the configuration of the mobile radio communication terminal 11 isdescribed below. The mobile radio communication terminal 11 includes acommunication module 30, an application execution unit 32, and a display34. The communication module 30 is a module that communicates with anexternal. radio communication terminal via a communication antenna 31according to a protocol based on the Bluetooth (registered trademark)standard. In the communication between the in-vehicle communication unit12 and the mobile radio communication terminal 11 in this embodiment,the in-vehicle communication unit 12 acts as a master (master unit) andthe mobile radio communication terminal 11 acts as a slave (slave unit).Once a slave ID is assigned and pairing with the in-vehiclecommunication unit 12 is established, the communication module 30responds only to a command to which the slave ID is assigned.

The communication module 30. makes variable the output to thecommunication antenna 31, for example, to change the effectivecommunication distance range. For example, to extend the communication.distance range, the communication module 30 increases the output to thecommunication antenna to increase the sending radio wave intensity;conversely, to reduce the communication distance range, thecommunication module 30 decreases the output to the communicationantenna to decrease the sending radio wave intensity.

The application execution unit 32 wirelessly communicates. with thein-vehicle communication unit 12 to execute an application that causesthe electronic key system to function. This application is installed bythe user in advance. The application execution unit 32 sends registeredID information 33 to the in-vehicle communication device 10 in responseto an ID request from the in-vehicle communication unit 12. In addition,when an output control signal from the in-vehicle communication unit 12is received via the communication module 30, the application executionunit 32 controls the output, sent from the communication module 30 tothe communication antenna 31, at a predetermined output level.

Next, with reference to FIG. 2 and FIG. 3, the following describes theoperation performed by the in-vehicle communication device 10 to measurethe distance to the mobile radio communication terminal 11. First, ifthe distance measuring start condition is satisfied, for example, if thevehicle 100 is parked, the in-vehicle device 15 starts the in-vehiclecommunication unit 12 described above. The in-vehicle communication unit12, once started, sends a signal for detecting a slave to detect anexternal radio communication terminal around the vehicle.

If external radio communication terminals 101, which are a slave, isdetected around the vehicle as shown in FIG. 2, the in-vehiclecommunication device 10 assigns a slave ID to the external radiocommunication terminal 101 and establishes pairing with the detectedexternal radio communication terminals 101. In this case, the in-vehiclecommunication device 10 can also acquire the profile that indicates thedevices provided on the external radio communication terminal 101 andthe, function of the terminal.

Next, as shown in FIG. 3, the, in-vehicle device 15 sends an ID request,which requests for the ID information 33 on the electronic key system,to one or more external radio communication terminals 101 via thein-vehicle communication unit 12 (step S1). At this time, if the profileis acquired as described above and the mobile radio communicationterminal 11 that has an application for the electronic key system isalready identified, the in-vehicle device 15 sends an ID request withthe slave ID of that terminal specified. Conversely, if the mobile radiocommunication terminal 11 that has an application for the electronic keysystem is not identified, the in-vehicle device 15 sends an ID requestto all external radio communication terminals 101.

When the ID request is received, the mobile radio communication terminal11 sends the ID information 33 to the in-vehicle device 15 with itsslave ID attached to the ID information 33 (step S2). For example, evenwhen an ID request is sent to all external radio communication terminals101 in step S1, only the mobile radio communication terminal 11 havingthe ID information 33 responds to the ID request.

When the ID information 33 is received, the in-vehicle device 15 checksthe ID information 33 using the ID checking unit 16 (step S3). If it isauthenticated as a result of the checking that the ID information 33 isvalid (step S4), the in-vehicle communication device 10 sends theabove-described output intensity maintaining request with the slave IDof the authenticated mobile radio communication terminal 11 specifiedfor the request (step S5).

At this time, the output intensity maintaining request is sent only tothe authenticated mobile radio communication terminal 11 as shown inFIG. 2. Therefore, the output control is not performed for the mobileradio communication terminal 11 that has no application for theelectronic key system and for the mobile radio communication terminal 11that has the application but is not authenticated. This preventsunnecessary output control from being performed for the external radiocommunication terminal 101.

As shown in FIG. 3, when the above-described output intensitymaintaining request is received via the communication module 30, theapplication execution unit 32 of the mobile radio communication terminal11 controls the communication module 30 and, with the output to thecommunication antenna 31 maintained at a predetermined value, sends theresponse signal to the in-vehicle communication device 10 as themeasuring signal (step S6).

When, the response signal is received, the in-vehicle device 15 measuresthe distance to the authenticated mobile radio communication terminal 11(step S7). To do so, the in-vehicle device 15 acquires the receptionradio wave intensity (for example, “−30 dB”) of the response signal,detected by the reception intensity detection unit 14, and measures thedistance as described above based on this reception radio waveintensity. After that, the in-vehicle device 15 determines whether themobile radio communication terminal 11 is within area Z that is within apredetermined distance from the vehicle 100 (see FIG. 2). For example,if the operation detection unit 17 detects a door handle operation whenthe mobile radio communication terminal 11 in area Z, the door lockcontrol unit 18 unlocks the door. For example, if the mobile radiocommunication terminal 11 gets out of area Z with the door unlocked, thedoor lock control unit 18 locks the door.

As described above, the radio communication system for vehicles and itsdistance measuring method in this embodiment give the following effects.(1) The in-vehicle device 15 sends the output intensity maintainingrequest, which sets the sending radio wave intensity to a pre-setintensity, to the authenticated mobile radio communication terminal 11.This request sets the sending radio wave intensity of the responsesignal, which is sent from the mobile radio communication terminal 11 tothe in-vehicle device 15, at a level. Therefore, even if the mobileradio communication terminal 11 has the general-purpose communicationfunction that makes variable the sending radio wave intensity, thismobile communication system makes it possible to accurately measure thedistance to the mobile radio communication terminal 11 using itsgeneral-purpose communication.

(2) The in-vehicle device 15 outputs the above-described outputintensity maintaining request only to the authenticated mobile radiocommunication terminal 11. This prevents unnecessary output control frombeing performed for the external radio communication terminal 101 thatis present around the vehicle but is not authenticated.

(Second Embodiment)

Next, a second embodiment of a radio communication system and itsdistance measuring method is described with emphasis on the differencefrom the first embodiment. Note that the basic configuration of theradio communication system and its distance measuring method in thisembodiment is similar to that of the first embodiment. In the drawings,the same reference numeral is given to substantially the same componentas that in the first embodiment, and the duplicated description will beomitted.

As shown in FIG. 4, the in-vehicle communication unit 12 of the vehicle100 may have a reception intensity detection unit for controlling theradio wave sending range but may have a reception intensity detectionunit omitted that is provided only for measuring the distance. Inaddition, the in-vehicle communication unit 12 has the function tocontrol the output to the in-vehicle communication antenna 13.

The communication module 30 of the mobile radio communication terminal11 has a reception intensity detection unit 35 that detects thereception radio wave intensity of a signal sent from the in-vehiclecommunication unit 12. In addition, as shown in FIG. 5, when the requestsignal is received from the in-vehicle device 15, the applicationexecution unit 32 of the mobile radio communication terminal 11 controlsthe reception intensity detection unit 35 to acquire the radio waveintensity of the received request signal. After that, using thecommunication module 30, the application execution unit 32 sends theinformation, which indicates the detected reception radio waveintensity, as well as the slave ID to the in-vehicle communicationdevice 10. The in-vehicle device 15 measures the distance based on theinformation on the reception radio wave intensity sent from the mobileradio communication terminal 11. That is, instead of detecting theintensity of the signal, sent from the electronic key terminal, on thevehicle side as in the conventional electronic key system, the radioWave intensity of the signal, sent from the vehicle 100 side andreceived by the mobile radio communication terminal 11, is fed back tothe in-vehicle communication device 10 for measuring the distance.

Next, with reference to FIG. 6, the distance measuring operationperformed by the in-vehicle communication device 10 in this embodimentto measure the distance to the mobile radio communication terminal 11 isdescribed below. The ID request (step S1) and the ID transmission (stepS2) are performed between the in-vehicle device 15 and the mobile radiocommunication terminal 11 as in the first embodiment.

After that, the in-vehicle device 15 performs the checking (step S3) andthe authentication (step S4) as in the first embodiment.

Next, via the in-vehicle communication unit 12, the in-vehicle device 15sends the request signal, which. requests for the reception radio waveintensity, to the authenticated mobile radio communication terminal 11as the distance measuring signal with the output to the in-vehiclecommunication antenna 13 set to a predetermined output level (step S20).

When the request signal is received, the application execution unit 32controls the communication module 30 to detect the reception radio waveintensity of the request signal (step S21). After that, the applicationexecution unit 32 sends the response signal, which includes theinformation on the detected reception radio wave intensity and the slaveID, to the in-vehicle device 15 (step S22).

When the response signal, which includes the information on thereception radio wave intensity, is received, the in-vehicle device 15references the value of the reception radio wave, intensity, included inthe response signal, and the table or the map described above andmeasures the distance to the mobile radio communication terminal 11(step S23).

In this manner, the mobile radio communication terminal 11, with nolimitation on the communication function that makes variable the sendingradio wave intensity, can unlock/lock the door and start the enginewhile maintaining the original general-purpose communication function.

As described above, the radio communication system for vehicles and itsdistance measuring method in this embodiment give the following effects.(3) The reception intensity of the radio wave, sent from the in-vehiclecommunication device 10 to the mobile radio communication terminal 11with a predetermined intensity, is detected by the mobile radiocommunication terminal 11 and its intensity information is returned tothe in-vehicle communication device 10. This method allows the vehicleside to measure the distance to the mobile radio communication terminal11 based on the value of the reception radio wave intensity withouthaving to control the sending radio wave intensity of the mobile radiocommunication terminal 11. Therefore, even if the mobile radiocommunication terminal 11 has the general-purpose communication functionthat makes variable the sending radio wave intensity, this mobilecommunication system makes it possible to accurately measure thedistance to the mobile radio communication terminal 11 using itsgeneral-purpose communication. In addition, the mobile radiocommunication terminal 11, which has no limitation on the sending radiowave intensity, can maintain its original general-purpose communicationfunction.

(Third Embodiment)

Next, a third embodiment of a radio communication system and itsdistance measuring method is described with emphasis on the difference,from the first embodiment. Note that the basic configuration of theradio communication system and its distance measuring method in thisembodiment is similar to that of the first embodiment. In the drawings,the same reference numeral is given to substantially the same componentas that in the first embodiment, and the duplicated description will beomitted.

As shown in FIG. 7, the mobile radio communication terminal 11communicates also with an output management device 50, an output controlunit capable of communication via Bluetooth (registered trademark), inthis embodiment. In the mobile radio communication terminal 11,identification information 51 is registered to indicate that theapplication for the electronic key system is installed. In other words,all mobile radio communication terminals 11, in which the applicationfor the electronic key system is installed, have this identificationinformation 51. The output management device 50 sends a message to thesurrounding external radio communication terminals 101 to ask whetherthey have the identification information 51. When the identificationinformation 51 is received from the external radio communicationterminal 101, the output management device 50 identifies that theapplication for the electronic key system is installed in that externalradio communication terminal 101.

In addition, the output management device 50 sends the output intensitymaintaining request to all mobile radio communication terminals 11 eachof which has the application for the electronic key system. This outputmanagement device 50, usually installed at the entrance of a parkinglot, controls the output of the mobile radio communication terminal 11before the user approaches the vehicle 100.

Next, with reference to FIG. 8, the distance measuring operationperformed by the in-vehicle communication device 10 in this embodimentto measure the distance to the mobile radio communication terminal 11 isdescribed below. The output management device 50 sends a request for theidentification information 51 to the surrounding external radiocommunication terminals 101 (step S30). The mobile radio communicationterminal 11, which has the identification information 51, sends itsidentification information 51 to the output management device 50 withthe slave ID attached (step S31).

The output management device 50 checks. the identification information51 for authentication (step S32). After successful authentication, theoutput management device 50 sends the above-described output intensitymaintaining request to the mobile radio communication terminal 11 fromwhich the identification information 51 is received (step S33). When theoutput intensity maintaining request is received, the mobile radiocommunication terminal 11 sets the output to the communication antenna31 to a predetermined output level.

After that, when the user carrying the mobile radio communicationterminal 11 approaches the vehicle 100, pairing is established betweenthe mobile radio communication terminal 11 and the in-vehiclecommunication unit 12. The in-vehicle device 15 sends the requestsignal, which requires a response signal, to the mobile radiocommunication terminal 11 (step S34). When the request signal isreceived, the mobile radio communication terminal 11 sends the responsesignal to the in-vehicle communication device 10 (step S35). Thereception intensity detection unit 14 of the in-vehicle communicationunit 12 detects the reception radio wave intensity of the responsesignal and measures the distance to the mobile radio communicationterminal 11 (step S7). After distance measuring is completed, theapplication execution unit 32 sends the output control release requestto the mobile radio communication terminal 11 via the communicationmodule 30 (step S36). When the output control release request isreceived, the mobile radio communication terminal 11 releases thelimitation on the output to the communication antenna 31 (step S37).

As described above, the radio communication system for vehicles and itsdistance measuring method in this embodiment give the effect, describedin (1) above, as well as the following effects.

(4) The output management device 50 sends the above-described outputcontrol signal to all mobile radio communication terminals 11 each ofwhich has the identification information 51. This at least prevents therequest, which sets the sending radio wave intensity to a predeterminedlevel, from being sent to the mobile radio communication terminal 11that has not the identification information 51. In this manner, theoutput management device 50 performs the output control of the mobileradio communication terminal 11 in advance before measuring thedistance, reducing the operation load on the in-vehicle device 15.

(Other Embodiments)

The above embodiments may be implemented also in the following mode.

-   -   In the second embodiment, if the request signal is received, the        application execution unit 32 detects the reception radio wave        intensity of the request signal. Instead of this, the request        signal, sent from the in-vehicle device 15, may include a        command, which detects the reception radio wave intensity and        sends the detected intensity back to the in-vehicle device 15,        so that this command detects the reception radio wave intensity.        -   The identification information 51 in the third embodiment            may be the ID information 33 on the electronic key system.            The condition for locking/unlocking the door of the vehicle            100 and for starting the engine is not limited to the            condition described in the embodiments described above but            may be changed. For example, the door may be unlocked only            on condition that the mobile radio communication terminal 11            is successfully authenticated or on condition that the            relative distance between the authenticated mobile radio            communication terminal 11 and the vehicle 100 is a            predetermined distance or shorter. In addition, the engine            may be started based not only on the operation to turn on            the engine switch 20 but also on the time the ignition            switch is turned on.        -   The in-vehicle communication unit 12 and the communication            module 30 of the mobile radio communication terminal 11 may            catty out communication based on the Wireless Fidelity            (Wi-Fi, registered trademark) Communication protocol            conforming to the IEEE.802.11 standard. In this case, the            in-vehicle communication unit 12 and the mobile radio            communication terminal 11 perform terminal-to-terminal            communication (peer-to-peer communication) that does not            require an intermediate node 5, such as a router. In            addition, the in-vehicle communication unit 12 and the            communication module 30 of the mobile radio communication            terminal 11 may carry out communication based on other radio            communication standards such as Infrared Data Association            (IrDA) or ZigBee (registered trademark).        -   The radio communication terminal that communicates with the            in-vehicle device 15 is not limited to a smartphone. The            radio communication terminal may be any device that has the            general-purpose communication function, for example, a music            player, a headphone, or a wrist watch may be used.        -   Although embodied in the electronic key system in this            embodiment, the radio communication system may embodied in            other systems that require distance measuring, for example,            in a system for starting the air conditioner in a vehicle            before riding in the vehicle or in a car finder system for            detecting the. position of the user's vehicle.

The invention claimed is:
 1. A radio communication system in which radiocommunication is carried out between an in-vehicle communication device,mounted in a vehicle, and a radio communication terminal to detect arelative distance therebetween, the radio communication systemcomprising: an output control unit configured to send an outputintensity maintaining request that requests a sending radio waveintensity of the radio communication terminal be maintained at apredetermined intensity; a communication portion, provided in the radiocommunication terminal, configured to: communicate according to aprotocol of general-purpose communication network, make variable thesending radio wave intensity, set the sending radio wave intensity atpredetermined intensity when the output intensity maintaining request isreceived, and send a measuring signal according to the protocol; and ameasuring unit, provided in the in-vehicle communication device,configured to calculate a distance to the radio communication terminalbased on a reception radio wave intensity of the measuring signal thatis sent from the radio communication terminal with the predeterminedintensity.
 2. The radio communication system according to claim 1,wherein the output control unit is further configured to send the outputintensity maintaining request to an authenticated radio communicationterminal.
 3. The radio communication system according to claim 1,wherein the output control unit, provided outside the vehicle, isfurther configured to send the output intensity maintaining request toall the radio communication terminals each of which has identificationinformation indicating that the radio communication terminal is capableof communicating with the in-vehicle communication device.
 4. The radiocommunication system according to claim 1, wherein the output controlunit is further configured to send a request, which requests for aprofile that indicates whether the radio communication terminal has anapplication for measuring between the in-vehicle communication deviceand the radio communication terminal, to the radio communicationterminal, the output control unit selects the radio communicationterminal that has the application based on the profile acquired from theradio communication terminal, the output control unit authenticate theradio communication terminal.
 5. The radio communication systemaccording to claim 1, wherein the communication portion is furtherconfigured to communicate based on a protocol of Bluetooth or a protocolof Wi-Fi.
 6. A distance measuring method for carrying out radiocommunication between an in-vehicle communication device, mounted in avehicle, and a radio communication terminal to detect a relativedistance therebetween, the distance measuring method comprising: sendingan output intensity maintaining request that requests a sending radiowave intensity of the radio communication terminal be maintained at apredetermined intensity, an output control unit provided in a portionother than the radio communication terminal, the radio communicationterminal communicating according to a protocol of general-purposecommunication network, making variable the sending radio wave intensity,setting the sending radio wave intensity at predetermined intensity whenthe output intensity maintaining request is received, sending ameasuring signal according to the protocol, and calculating a distanceto the radio communication terminal based on a reception radio waveintensity of the measuring signal that is sent from the radiocommunication terminal with the predetermined intensity.